In recent years, applications of polarizing plates or phase contrast plates employing photopolymerizable liquid crystals have been proposed as optical elements.
Such optical elements are obtained by liquid crystal-state polymerization and fixing of polymerizing liquid crystals with optical anisotropy. By accomplishing polymerization after appropriate control of orientation in the liquid crystal state, it is possible to obtain polymers having various modes of optical anisotropy such as homogeneous orientation, tilt orientation, hybrid orientation, homeotropic orientation and twist orientation.
Polymers with homogeneous orientation have been applied in combination with, for example, ½ wavelength plates, ¼ wavelength plates or films with other optical functions. Polymers with hybrid orientation have been applied, for example, as view angle compensation plates in TN (Twisted Nematic) mode. Polymers with homeotropic orientation have been applied in combination with, for example, films having other optical functions for enhancement of view angle characteristics of polarizing plates.
Also, polymers with twisted orientation, i.e. cholesteric liquid crystal polymers composed mainly of polymerizable liquid crystal molecules with positive birefringence, have been applied as various optical elements based on their helical pitch (P). For visible light applications, a pitch P sufficiently greater than the wavelength permits, for example, applications in head-up displays and projectors utilizing the rotatory function, or applications for optical compensation in ST (Super Twisted Nematic) mode, using the birefringence function (see Patent document 1).
When P is approximately equal to the wavelength, applications are largely divided into (A) 350/nave (nm)<P800/nave (nm) and (B) P350/nave (nm). Here, nave is the mean refractive index. In the case of (A), for example, applications include utilizing selective reflection for purposes of design with decorative materials, anti-counterfeit measures and color filters used in liquid crystal display elements, or applications in brightness-enhanced films (see Non-patent document 1).
In the case of (B), the refractive index of the visible light range on the surface orthogonal to the helical axis is represented by (ne2+no2)/2)0.5, and the refractive index of the visible light range in the helical axis direction is equal to no (see Non-patent document 2). Optical films with such optical properties are known as negative C-plates. Negative C-plates can compensate for retardation of liquid crystal regions with positive birefringence oriented orthogonal to the liquid crystal cell, and can therefore serve as compensation plates suited for enhanced viewing angle properties in liquid crystal display elements, particularly VA (Vertically Aligned), TN (Twisted Nematic), OCB (Optically Compensated Birefringence) and HAN (Hybrid Aligned Nematic) display elements. At the current time, these employ drawn polymer films or films utilizing discotic liquid crystals having planar oriented negative birefringence (see Patent document 2). Utilizing a cholesteric liquid crystal polymer made of liquid crystal molecules with positive birefringence increases the degree of design freedom for the refractive index anisotropy and its wavelength dispersion. Negative C-plates can also be used in combination with various compensation layers.
For all of the uses mentioned above, the optical anisotropy film may be provided either inside the cell or outside the cell. An example of a system where it is provided inside the cell is the system described in Patent document 3. An example of a system where it is provided outside the cell is the system described in JP-A-H14-372623/2002. When it is provided outside the cell, the polymerizable liquid crystal material is often laminated on a film such as TAC (triacetyl cellulose) or norbornene resin, as the support.
When the polymer of a polymerizable liquid crystal is utilized for the structure described above, a sufficient degree of heat resistance is required for use inside the cell, due to the conditions for molding of the other members, while for use outside the cell, satisfactory adhesion with the support and sufficient heat and humidity resistance are required. In both cases, it has been desirable to develop a photopolymerizable liquid crystal composition which, in terms of properties before curing, has a nematic phase at room temperature, has a wide nematic phase, exhibits satisfactory orientation properties and has rapid curing properties by UV irradiation in air, and in terms of properties after curing, has a suitable Δn for optical designs, has transparency and exhibits excellent heat and humidity resistance.
Patent document 1: JP-A-H8-87008/1996
Patent document 2: JP-A-H14-6138/2002
Patent document 3: JP-A-H13-222009/2001
Non-patent document 1: Y. Hisatake et al, Asia Display/IDW '01 LCT8-2
Non-patent document 2: W. H. de Jeu, Physical Properties of Liquid Crystalline Materials, Gordon and Breach, New York (1980)